service-worker.js

import * as sapper from '@sapper/service-worker';

const ASSETS = `cache${sapper.timestamp}`;

// `shell` is an array of all the files generated by webpack,
// `files` is an array of everything in the `static` directory
const to_cache = sapper.shell.concat(sapper.files);
const cached = new Set(to_cache);

self.addEventListener('install', event => {
	event.waitUntil(
		caches
			.open(ASSETS)
			.then(cache => cache.addAll(to_cache))
			.then(() => {
				self.skipWaiting();
			})
	);
});

self.addEventListener('activate', event => {
	event.waitUntil(
		caches.keys().then(async keys => {
			// delete old caches
			for (const key of keys) {
				if (key !== ASSETS) await caches.delete(key);
			}

			self.clients.claim();
		})
	);
});

self.addEventListener('fetch', event => {
	if (event.request.method !== 'GET') return;

	const url = new URL(event.request.url);

	// don't try to handle e.g. data: URIs
	if (!url.protocol.startsWith('http')) return;

	// ignore dev server requests
	if (url.hostname === self.location.hostname && url.port !== self.location.port) return;

	// always serve assets and webpack-generated files from cache
	if (url.host === self.location.host && cached.has(url.pathname)) {
		event.respondWith(caches.match(event.request));
		return;
	}

	// for pages, you might want to serve a shell `index.html` file,
	// which Sapper has generated for you. It's not right for every
	// app, but if it's right for yours then uncomment this section
	/*
	if (url.origin === self.origin && routes.find(route => route.pattern.test(url.pathname))) {
		event.respondWith(caches.match('/index.html'));
		return;
	}
	*/

	if (event.request.cache === 'only-if-cached') return;

	// for everything else, try the network first, falling back to
	// cache if the user is offline. (If the pages never change, you
	// might prefer a cache-first approach to a network-first one.)
	event.respondWith(
		caches
			.open(`offline${sapper.timestamp}`)
			.then(async cache => {
				try {
					const response = await fetch(event.request);
					cache.put(event.request, response.clone());
					return response;
				} catch(err) {
					const response = await cache.match(event.request);
					if (response) return response;

					throw err;
				}
			})
	);
});